The present disclosure relates generally to the field of power supplies for information handling systems, and more particularly to techniques for efficiently providing power to drive a discharge lamp, such as a cold cathode fluorescent lamp (CCFL).
As the value and use of information continues to increase, individuals and businesses seek additional ways to acquire, process and store information. One option available to users is information handling systems. An information handling system (‘IHS’) generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.
Liquid crystal display (LCD) panel based display devices have been commonly utilized in many IHS systems due to their compact size, and low power consumption. Although there are different types of backlights (e.g., light sources including a discharge lamp), which are currently used for backlighting the latest LCD panels, the CCFL (also known as cold cathode fluorescent tube (CCFT)) is most commonly used. Circuits for supplying power to CCFL's generally require a controllable alternating current (AC) power supply and a feedback loop to accurately monitor the current in the lamp in order to maintain operating stability of the circuit and to have an ability to vary the lamp brightness. Such circuits typically generate a high voltage to initially turn on the CCFL and then lower the voltage when current begins to flow through the lamp. For example, Monolithic Power Systems, Inc., located at 983 University Ave, Building D, Los Gatos, Calif. 95032, USA, provides a MP1015 power circuit chip for driving the CCFL.
Such circuits also typically include an inverter circuit to convert a direct current (DC) voltage received as an input to a regulated AC voltage generated as an output. Inverter circuits typically include a controller component, such as a pulse width modulator (PWM) based controller. Various well-known inverter circuit configurations or “topologies” include a Royer converter, full-bridge or half-bridge inverters.
The CCFL power consumption may account for a significant portion (e.g., up to 50% in some cases) of the IHS system power requirement, especially for portable systems. Therefore, there is a considerable amount of interest to achieve advantages in extending battery life and reducing re-charge frequency by improving the efficiency of power supplies configured to provide power to the CCFL.
In traditional inverter based power circuits, changes in input voltage cause a decrease in power conversion efficiency. For example, when an adapter is unplugged/plugged into a portable IHS system such as a notebook computer, the voltage to the LCD backlight inverter varies from AC adapter voltage (approximately between 18V and 22V) to the battery voltage (approximately between 9V and 17V). The varying voltage causes a noticeable change in the LCD brightness level, which is often perceived as a flicker.
Brightness output from the CCFL is a function of operating frequency. FIG. 1A illustrates a graphical relationship between LCD brightness (shown on Y-axis) versus frequency (shown on X-axis) measurements for a commercially available CCFL. Curves 110, 120 and 130 are shown for lamp currents of 6 mA, 5 mA and 4 mA.
Power conversion efficiency of an inverter is a function of the input voltage. FIG. 1B illustrates a graphical relationship between a change in efficiency (shown on Y-axis) versus input voltage (shown on X-axis) measurements for a commercially available inverter.
FIG. 1C illustrates a graphical relationship between a change in frequency (shown on Y-axis) versus input voltage (shown on X-axis) measurements for a commercially available inverter. Thus, frequency of the backlight inverter changes as a function of input voltage causing the brightness change and the change in frequency causes a decrease in the inverter efficiency.
Therefore, a need exists for improved efficiency of the power circuits providing power to the CCFL. More specifically, a need exists to develop tools and techniques for improving the efficiency of inverters under changing voltage and frequency conditions. Accordingly, it would be desirable to provide tools and techniques for an improved inverter of an IHS absent the disadvantages found in the prior methods discussed above.